This invention relates to methods and apparatus for recovering reaction products from surfaces. The invention may be applied to recovering a reaction product formed at the surface of an electrode in an electrochemical cell. The invention may also be applied to recovering a product of a catalysed chemical reaction from the surface of a catalyst. The invention has particular application in collecting reaction products generated at the anodes or cathodes of electrochemical cells used for electrical refining and electrowinning of metals. Another aspect of the invention relates to electrochemical cells adapted to be operated according to the methods of the invention.
In some industrial chemical processes, chemical reaction products are produced at electrodes which are immersed in a bath of electrolyte as part of an electrochemical cell. It is often desirable to remove such reaction products before they can intermingle with other species in the electrolyte. The reaction products may be desirable in their own right or it may be necessary to remove them from the cell to prevent them from interfering with other chemical processes taking place within the cell.
For example, some processes for producing copper metal involve leaching copper sulphide ores with an acidic solution containing an oxidizing agent such as ferric ions (Fe3+). The copper dissolves leaving behind elemental sulfur by the reaction:
Cu2S+4Fe3+xe2x88x922Cu2++4Fe2++S0xe2x80x83xe2x80x83(1) 
Copper metal is then recovered from the resulting copper-containing solution by an electrowinning step. The electrowinning step takes place in an electrochemical cell containing a cathode, an anode and an electrolyte containing the cupric (Cu2+) ions. When electrical current is passed through the cell from the cathode to the anode, copper metal is plated onto the cathode by a process involving the reaction:
Cu2++2exe2x88x92xe2x88x92Cuxe2x80x83xe2x80x83(2) 
At the anode, ferrous (Fe2+) ions are converted to ferric ions by the reaction:
Fe2+xe2x88x92Fe3xe2x88x92+exe2x88x92xe2x80x83xe2x80x83(3) 
If the ferric ions so created are allowed to co-mingle with the electrolyte then they may ultimately return to the cathode where they will be converted back to ferrous (Fe2+) ions. This reduces the efficiency with which copper metal is plated onto the cathode. The inventors have recognized that this also results in a loss of opportunity to use the ferric ions in other parts of the overall process.
There are various means currently being used to prevent species produced at an electrode in an electrochemical cell from interfering with electrochemical processes taking place at the other electrode. One approach is to use a divided cell. A divided cell has a barrier between the electrodes. The barrier is permeable to charge carriers but has a low permeability to the species in question, The barrier nay comprise a suitable diaphragm or membrane. A disadvantage of divided cells is that the membrane introduces electrical resistance which increases power consumption. The membrane also adds both capital and maintenance expense, can degrade over time and may become plugged.
In some cases a reaction product produced at an electrode in an electrochemical cell is a desired product which should be recovered. The prior art provides various ways to recover such reaction products. In general, these methods all make it necessary to withdraw a substantial amount of electrolyte, together with the reaction product which is being collected. For example, in cells where the desired reaction product is much less dense than the electrolyte then the desired reaction product may be obtained by withdrawing the top portion of the electrolyte. In some prior art devices, the electrodes are themselves porous, thereby allowing the reaction products produced at the electrodes, together with some electrolyte, to be withdrawn through the electrodes. Porous electrodes are susceptible to becoming plugged with small particles of sediment or the like which are often present in electrochemical cells. In general, all of these processes allow the desired species to mix with a substantial quantity of electrolyte as it is being removed. Therefore, the concentration of the desired reaction product removed from the cell is relatively low. Subsequent concentration steps are often required.
Another example of a situation where a desired species is created at a surface is a process which uses a solid catalyst to promote a reaction involving species in the fluid to produce a reaction product at the surface of the catalyst.
There exist systems for water hydrolysis which have a pair of very closely spaced electrodes. Water flows between the electrodes. The close spacing of the electrodes forces laminar flow of the water between the electrodes. Acidic water can be drawn off through a slit. Such systems could not be effectively used in industrial settings where the very narrow space between the electrodes would be susceptible to becoming plugged with debris. Examples of such systems are disclosed in Patent Abstracts of Japan No. JP-A-08168767 and U.S. Pat. No. 5,534,120.
There is a continuing need for methods and apparatus capable of efficiently collecting dissolved reaction products produced at a surface in an electrochemical cell or a catalyst. There is a particular need for such apparatus and methods which can be practised on an industrial scale with a minimum of maintenance.
This invention creates a condition in which an electrolyte, or other fluid, flows in a thin boundary layer across a surface at which a reaction product is generated. The reaction product enters the fluid flowing in the boundary layer and is carried by the flowing fluid to a collection zone. Fluid containing the reaction product is removed at the collection zone. Various flow conditions in which the boundary layer does not mix significantly with the bulk of fluid adjacent the surface may be used to practise the invention.
A specific embodiment of the invention provides a method for recovering a reaction product at a surface. The method comprises the steps of: providing a surface immersed in a fluid; causing the fluid to flow past the surface so as to cause fluid in a boundary layer region adjacent the surface to flow across the surface from an upstream side of the surface to a downstream side of the surface without mixing substantially with fluid from outside of the boundary layer region; allowing a reaction product to form at the surface; allowing the fluid flowing in the boundary layer region to carry the reaction product across the surface toward a downstream collection zone; and, at the collection zone, collecting fluid flowing in the boundary layer region together with the reaction product being carried by the fluid flowing in the boundary layer region. By applying this method the collected reaction product can be obtained in a form which is much more concentrated than would be the case it the reaction product was simply allowed to mix with the fluid.
In the method of the invention the surface may be a surface of a catalyst where the reaction product is formed by a chemical reaction catalysed by the catalyst. In the alternative, the surface could be a surface of an electrode where the reaction product is formed by an electrochemical reaction. In one exemplary preferred embodiment the reaction product comprises ferric ions produced at an electrode surface by oxidation of ferrous ions.
The step of collecting the fluid flowing in the boundary layer region preferably comprises drawing fluid flowing in the boundary layer region into an elongated aperture extending transversely to the direction of fluid flow as the fluid flowing in the boundary layer region arrives at the collection zone. Most preferably the elongated aperture comprises an elongated slot extending transversely to a direction of the boundary layer flow.
Another aspect of the invention provides a method for recovering a reaction product at a surface. The method comprises the steps of: providing a surface immersed in a fluid and fluid driving means for causing the fluid to flow past the surface; operating the fluid driving means to cause the fluid to flow past the surface so as to cause fluid in a boundary layer region adjacent the surface to flow across the surface from an upstream side of the surface to a downstream side of the surface without mixing substantially with fluid outside of the boundary layer region; allowing a reaction product to form at the surface; allowing the fluid flowing in the boundary layer region to carry the reaction product across the surface toward a fluid removal means in a downstream collection zone; and, at the collection zone, operating the fluid removal means to withdraw fluid flowing in the boundary layer together with the reaction product.
Yet another aspect of the invention provides an electrochemical cell comprising: a container holding a liquid electrolyte; first and second electrodes in the container in contact with the electrolyte; electrical conductors for connecting the first and second electrodes respectively to first and second poles of an electrical power supply; fluid driving means for causing electrolyte to flow past a surface of the first electrode with a velocity sufficient to cause the electrolyte in a boundary layer adjacent the surface to flow across the first electrode from an upstream side to a downstream side without mixing significantly with electrolyte from outside the boundary layer; and, fluid removal means at the downstream side of the surface for withdrawing fluid containing a reaction product from the boundary layer.
The fluid removal means preferably comprises an elongated parallel-sided slot oriented generally transversely to a direction of fluid flow in the boundary layer. The slot most preferably has a width in the range of about 1 millimeter to about 1.5 millimeters. An opening according to preferred embodiments of the invention is much more resistant to becoming plugged with debris than a prior art porous electrode.
A further aspect of the invention comprises an electrode assembly for use in an electrochemical cell. The electrode assembly comprises an electrically conductive surface; a first elongated opening extending along an edge portion of the surface; a plenum in fluid communication with the first elongated opening; and, a port connected to the plenum. When the electrode assembly is immersed in an electrolyte, fluid can be drawn into the first elongated opening by applying suction to the port. In some applications it is preferable to provide one or more additional elongated openings in fluid connection with the plenum. The additional elongated openings extend generally parallel to the first elongated opening and are spaced apart on the electrically conductive surface. Withdrawing fluid through the additional openings can help to stabilize flow in a boundary layer adjacent the surface. The additional openings also permit removal of depleted fluid at various points across the electrically conductive surface .